It is an object of the present invention to provide a substrate for lithium ion battery separators which has high adhesion to an inorganic particle layer, can be made thin and is excellent in tensile strength and cuttability and a lithium ion battery separator including the substrate for lithium ion battery separators. The substrate for lithium ion battery separators which contains heat-resistant fibers and synthetic resin short fibers contains fibrillated heat-resistant fibers having a modified freeness of not more than 300 ml as the heat-resistant fibers and has a content of the fibrillated heat-resistant fibers having a modified freeness of not more than 300 ml of not less than 1.0 mass % to less than 5.0 mass % based on the total of all the fiber components contained in the substrate. The modified freeness is a value measured in accordance with JIS P8121-2:2012 except that an 80-mesh wire net having a wire diameter of 0.14 mm and an opening of 0.18 mm is used as a screening plate and the concentration of a sample is 0.1%.